Creation Kit - Adding Custom Creatures

Note that you can carefully manipulate the NIF object to add new blocks as needed. As long as the hierarchy doesn't change you should be able to save it without having to go back into 3DS max and start the process again.

Step 2 - Add the following forms with these instructions

ArmorAddon - Find the form of the creature's skeleton that you're using and duplicate it. Then replace the model references with your model that was exported.

Race - (Optional) Add a new race using the original creature's race as reference. You may want to select the original race under "Copied Data" if it is available if you are using a new race but refer the armor to a different race.

Armor - Find the form of the creature skin that references the ArmorAddon you copied. Make your own and change it accordingly.

Actor - Create the actor and reference your new Race and Skin (armor)

Troubleshooting
If your NPC is invisible in Creation Kit or in game there are two main issues. The first is that you have not properly matched the race setting in the Armor and ArmorAddon. Ensure that the race setting is consistent in the NPC, Armor and ArmorAddon. If this doesn't work try setting the ArmorAddon Biped back to a vanilla mesh with the same skeleton then press OK. Try adding your NPC to a cell and select it to bring up the properties dialogue box. If the model is visible it means the NIF file is at fault. You may be able to export the NIF without adding BSDismemberSkinModifier and merely relying on NiSkinInstance nodes instead.